The present invention relates to an image signal processor for a facsimile device, a copying machine, etc., and more particularly, to an image signal processor which performs a proper image process for each type of image signal. When a digitized input image signal of a manuscript in which halftone images of a character, a photograph and a screen halftone are mixed is uniformly binarized, the gradation of the halftone images is not reproduced. When a quasi-tone reproduction process, which is represented by a dither process, is uniformly executed for the image input signals, characters become obscure or blurred. This is because of the fact that the binarization is originally a process suitable for a character, and a quasi-tone reproduction process is suitable for a halftone image. It is therefore necessary to execute a proper process upon identifying the type of an input image. An image processor having an image signal identification device which accurately identifies the type of input image is needed for this purpose.
In a Japanese patent application laid-open No. 80965/1986, an image identification method is disclosed as follows. Inside a block surrounding a picture element to be identified, a plurality of density difference values between pairs of fixed picture elements is detected, and if a maximum value among these density gradient values is larger than a certain threshold value, the picture element is identified to be a character, and if the maximum value of density gradient values is smaller than the certain threshold value, the picture element is identified to be a halftone image.
When a method of this kind is used, there is a possibility that a screen halftone will be mistaken for a character. Since a screen halftone is originally an image recorded with white and black dots on a sheet of paper, when a manuscript is read, the maximum value of the density difference in a block can be as large as that of a character. It is therefore difficult to distinguish a character from a screen halftone based solely on the density difference. Furthermore, when a screen halftone is dither-processed, moire stripes (a periodic density change which is not found in a manuscript) are produced.